Norris had looked to have extended his lead to 30 points ahead of the penultimate round in Qatar after finishing runner-up to Max Verstappen on Sunday (AEDT), while Piastri settled for fourth.
But the McLaren pair - both hunting a maiden title - were stripped of their results more than four hours after the race finished.
Stewards had found the rearmost skids underneath the cars were less than the 9mm minimum thickness required
The rulings mean Norris's championship lead has been trimmed to 24 points, with Verstappen moving to equal second alongside Piastri in the standings with two rounds remaining.
Verstappen had produced a flawless drive across the 50-lap race to keep his campaign for a fifth-straight title alive, claiming his sixth win of the year with a monster 20.741-second buffer against Norris.
The Red Bull star had started second on the grid but took the lead from polesitter Norris after a mistake from the British driver on the opening lap.
It was his 69th-career victory and eighth consecutive podium.
Mercedes driver George Russell was third after fierce attempts to usurp Norris, with rookie teammate Kimi Antonelli crossing the line in fourth.
A five-second penalty to Antonelli for a false start promoted Piastri to fourth.
Piastri has now failed to podium in his past six races, after leading the championship by a season-high 34 points following the Dutch Grand Prix in August.
"I don't really know what to think," Piastri told Sky Sports before being disqualified.
"Obviously, I need more than (cutting into the lead) now.
"The championship picture is what it is. We'll see what I can do."
Verstappen was similarly wary of getting ahead of himself.
"It's still a big gap, but we always just try to maximise everything that we've got and this weekend, that was first," Verstappen said.
Norris had brilliantly defended against Verstappen off the starting line but went too wide on the way to turn one, with the gap also allowing compatriot Russell to sneak into second.
"I let Max have a win. I let him go, let him have a nice race," Norris joked.
"No, I just braked too late - it was my F-up."
Piastri was just as unlucky in a chaotic start, making contact with New Zealand's Liam Lawson before tumbling to seventh.
Racing Bull's Lawson had suffered heavy damage to his front right wing and finished 16th after qualifying sixth.
Piastri escaped sanction for the collision, with Lawson appearing to turn into the side of the McLaren after braking heavily to avoid Russell.
The Melburnian had been handed a controversial 10-second penalty a fortnight ago in Brazil for a three-way crash with Antonelli and Charles Leclerc, in a huge blow to his title hopes.
Sauber rookie Gabriel Bortoleto and Aston Martin driver Lance Stroll did not cross the finish line in Las Vegas after a race-ending collision on the opening lap.
Bortoleto is set to come under investigation for making contact with Stroll at the first corner.
Ferrari driver Lewis Hamilton recovered from 19th following a disastrous qualifying performance to finish 10th, but the seven-time world champion cut a dejected figure.
"It's been the worst season ever, and no matter how much I try, it just keeps getting worse," Hamilton said.
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